Maybe the cruelty is the point?

They can cause issues while transiting through and they are required to give a shit because they're part of the EU's outer border control. And they might have fears of some of the migrants staying. I could imagine someone being in the coastguard cares about securing the border too even if there were none of the above issues.

Because of European asylum rules. Those migrants have to be processed in their country of entry.

Also, because they are racist fucks, who are paid to believe that Greece is being invaded.